Processes for imparting water-repellency to wood substrates using water-based formulations, i.e. oil-in-water emulsions, are well known. Generally, such formulations may be applied by dip, brush or spray, but the modern trend is to impregnate the wood by means of a pressure process.
Water repellents have only a slight effect on the rate of absorption of water vapor in timber, but they can be very effective in reducing absorption of liquid water. They have no effect on the equilibrium moisture content of wood. The object of water repellent treatment of wood is to reduce the wettability of the wood surface so that liquid water does not form a coherent film and does not penetrate the surface structure between boards, and especially the permeable end grain. The process of the invention results in conferring superior water repellency to wood, thereby preventing the absorption of liquid water and providing a degree of dimensional stability and preventing rapid swelling and shrinkage during wetting and drying and is also effective in reducing the rate of mechanical degradation, surface checking and cracking in treated wood during initial drying or in service.
Typically, wood preservatives such as salts based on copper-chromium-arsenic are incorporated in the water-based formulations which provide water repellency to the wood in order to also impart resistance to fungal or insect attack to the wood.
Since the water-based formulations are emulsions of the oil-in-water type, the formulations will contain one or more surfactants to provide stability to the emulsion. Typically, such emulsions will contain one or more surfactants of the nonionic type.
Prior art processes for imparting water repellency to lumber generally are satisfactory when the lumber is derived from a species such as Southern Yellow pine or Radiata pine. However, prior art processes for imparting water repellency are unsatisfactory when the lumber is derived from a refractory species such as Ponderosa pine, Jack pine, Scots pine or Hem-fir, since there is an unacceptably low penetration of the water-based formulation (and any preservative that may be contained in the formulation) into such lumber.
The unsatisfactory penetration of water-based water repellent formulations, i.e. oil-in-water emulsions, into refractory wood species was previously believed to be due to the large particle size of the emulsions. However, it has now been discovered that the shear stability of the emulsion is critical to obtain a satisfactory degree of wood penetration, especially in refractory species.